[clang] [Sema] Fix missing warnings for unused args with invalid printf specs. (PR #158514)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

I'm not sure we want this more chatty? It is kind of obvious that for an invalid format specifier the data argument wont be used. Adding more reviewers

https://github.com/llvm/llvm-project/pull/158514